Role of Mn2+ Doping in the Preparation of Core-Shell Structured Fe3O4@upconversion Nanoparticles and Their Applications in T1/T2-Weighted Magnetic Resonance Imaging, Upconversion Luminescent Imaging and Near-Infrared Activated Photodynamic Therapy

Core-shell (C/S) structured upconversion coated Fe3O4 nanoparticles (NPs) are of great interest due to their potential as magnetic resonance imaging (MRI) and upconversion luminescent (UCL) imaging agents, as well as near-infrared activated photodynamic therapy (PDT) platforms. When C/S structured Fe3O4@Mn2+-doped NaYF4:Yb/Er NPs were prepared previously, well-defined C/S-NPs could not be formed without the doping of Mn2+ during synthesis. Here, the role of Mn2+ doping on the synthesis of core-shell structured magnetic-upconversion nanoparticles (MUCNPs) is investigated in detail. Core-shell-shell nanoparticles (C/S/S-MUCNPs) with Fe3O4 as the core, an inert layer of Mn2+-doped NaYF4 and an outer shell consisting of Mn2+-doped NaYF4:Yb/Er were prepared. To further develop C/S/S-MUCNPs applications in the biological field, amphiphilic poly(maleic anhydride-alt-1-octadecene) (C18PMH) modified with amine functionalized methoxy poly(ethylene glycol) (C18PMH-mPEG) was used as a capping ligand to modify the surface of C/S/S-MUCNPs to improve biocompatibility. UCL imaging, T1-weighted MRI ascribed to the Mn2+ ions and T2-weighted MRI ascribed to the Fe3O4 core of C/S/S-MUCNPs were then evaluated. Finally, chlorine e6 (Ce6) was loaded on the C/S/S-MUCNPs and the PDT performance of these NPs was explored. Mn2+ doping is an effective method to control the formation of core-shell structured MUCNPs, which would be potential candidate as multifunctional nanoprobes for future T1/T2-weighted MR/UCL imaging and PDT platforms.